How does $\mathbb{E}$ suddenly change to $\mathbb{E}_{\pi'}$ in this equation?
Also, in general, in the conditional expectation, which distribution do we compute the expectation with respect to? From what I have seen, in $\mathbb{E}[X|Y]$, we always calculate the expected value over distribution $X$.


No, for $\mathbb{E}[X|Y]$ we take expectation of $X$ with respect to the conditional distribution $X|Y$, i.e. 

$$\mathbb{E}[X|Y] = \int_\mathbb{R} x p(x|y)dx\;;$$

where $p(x|y)$ is the density function of the conditional distribution. If your random variables are discrete then replace the integral with a summation. Also note that $\mathbb{E}[X|Y]$ is still a random variable in $Y$. 


  How does $\mathbb{E}$ suddenly change to $\mathbb{E}_{\pi '}$ and the $A_t = \pi '(s)$ term disappears? 


This is because in this instance $\pi '(s)$ is a  deterministic policy, i.e. in state $s$ the policy will take action $b$ with probability 1 and all other actions with probability 0. NB: this is the convention used in Sutton and Barto to denote a deterministic policy.

Without loss of generality, assume that $\pi'(s) = b$. The implication of this is that in the first expectation we have 
$$\mathbb{E}[R_{t+1} + \gamma v(S_{t+1}) | S_t = s, A_t = \pi'(s) = b] = \sum_{s',r}p(s',r|s,a=b)(r + \gamma v(s'))\;,$$
and in the second expectation we have 
$$\mathbb{E}_{\pi'}[R_{t+1} + \gamma v(S_{t+1}) | S_t = s] = \sum_a\pi'(a|s)\sum_{s',r}p(s',r|s,a)(r + \gamma v(s'))\;;$$
However, we know that $\pi'(a|s) = 0 \; \forall a \neq b$, so this sum over $a$ would equal 0 for all $a$ except when $a=b$, in which case we know that $\pi'(b|s) = 1$, and so the expectation becomes 

$$\mathbb{E}_{\pi'}[R_{t+1} + \gamma v(S_{t+1}) | S_t = s] = \sum_{s',r}p(s',r|s,a=b)(r + \gamma v(s'))\;;$$

and so we have equality of the two expectations.